George W. Bush

Today, the Environmental Protection Agency proposed a major rule to regulate power plants under the Hazardous Air Pollutants (HAP) Section 112 of the Clean Air Act.

This post is a primer on this consequential and controversial decision.

Section 112 of the Clean Air Act

In 1970, the Congress added Section 112 to the Clean Air Act, requiring that the EPA list and regulate Hazardous Air Pollutants (HAPs) that could “cause, or contribute to, an increase in mortality or an increase in serious irreversible or incapacitating reversible illness.” The Congress ordered the EPA to establish standards for HAPs that provided “an ample margin of safety to protect public health.”

Due to difficulties interpreting what should constitute “an ample margin of safety,” the EPA largely ignored Section 112 for two decades.

In 1990, the Congress, frustrated with the slow pace of HAP regulation, amended the Clean Air Act to remove much of EPA’s discretion over the implementation of Section 112. Lawmakers listed 189 pollutants for regulation. They also legislated HAP pollution controls, known as Maximum Achievable Control Technology (MACT) standards. The Clean Air Act amendments set a “MACT floor” (i.e., a minimum HAP pollution control) at “the average emission limitation achieved by the best performing 12 percent of the existing sources.”

Section 112 MACT standards apply to both new and existing stationary sources.

Notably, the Congress required the EPA to proceed with caution before it regulated Electricity Generating Units (“EGUs,” or power plants). The 1990 Clean Air Amendments mandated a study on the public health threats posed by EGU HAP emissions, and the EPA Administrator was authorized to proceed with the regulation of HAPs from EGUs only after evaluating the results of this study, and concluding that “such regulation is appropriate and necessary.”